1. Field of the Invention
This invention relates to a vibration wave driven motor in which a movable member is frictionally driven by a vibration member and, more particularly, to the structure of an electricity supply means for such a motor.
2. Description of the Related Art
Vibration wave driven motors, in which a vibrational motion of an electro-mechanical energy conversion element such as a piezoelectric element caused-when an alternating wave voltage is applied to the element is converted into a rotational motion or one-dimensional motion, have a simpler structure and a smaller size in comparison with conventional electromagnetic motors because they require no coil winding. Also, they are capable of obtaining a large torque even at a low rotational speed. They have attracted attention in recent years because of these advantages.
FIGS. 9 and 10 show the principle of driving of a vibration wave driven motor, and FIG. 9 shows vibration waves generated in a vibration member of the motor. Piezoelectric elements 2a and 2b, arranged into a piezoelectric plate 2, form electro-mechanical energy conversion elements and are bonded to a vibrating member 1 (ordinarily, a metallic member). Elements 2a and 2b are arranged in positions relatively shifted .lambda./4 where .lambda. is the wavelength in the vibrating member. The piezoelectric elements 2a and 2b and the vibrating member 1 constitute a stator.
In the case of a conventional motor, a common electrode is provided on one of two surfaces of each of piezoelectric elements 2a and 2b in contact with the vibrating member 1 which is conductive, and an electrode is provided on the other surface of each piezoelectric element 2a or 2b. Each element is previously polarized as indicated by .sym. and .crclbar. in FIG. 9. An AC voltage of V=Vosin(.OMEGA.t.+-..pi./2) is applied to the piezoelectric element 2a from an AC power source 3a, while an AC voltage of V=Vosin(.omega.t.+-..pi./2) phase-shifted by .pi./2 is applied to the piezoelectric element 2b through a 90.degree. phase shifter 3b. (+) and (-) in the above equation are changed over by the phase shifter 3b according to the direction in which the movable member 6 is moved. It is assumed here that (-) is selected and that a voltage of V=Vosin(.omega.t-.pi./2) is being applied to the piezoelectric element 2b. If the piezoelectric element 2a is oscillated alone by the voltage V=Vosin .omega.t, vibration of a standing wave such as that shown in (a) of FIG. 9 occurs. If only the piezoelectric element 2b is oscillated by the voltage V=Vosin (.omega.t-.pi./2), vibration of a standing wave such as that shown in (b) of FIG. 9 occurs. When these two AC voltages out of phase with each other are simultaneously applied to the piezoelectric elements 2a and 2b, the vibration wave becomes a traveling wave. A wave shown in (A) of FIG. 9 is exhibited at a time t=2n.pi./.OMEGA., a wave in (B) is exhibited at a time t=.pi./2.omega.+2n.pi./.omega., a wave in (C) is exhibited at a time t=.pi./.omega.+2n.pi./.omega., and a wave in (D) is exhibited at a time t=3.pi./2.omega.+2n.pi./.omega.. The wave front of the vibration wave advances in the direction x (FIG. 10).
This traveling vibration wave involves a longitudinal wave and a transverse wave. As shown in FIG. 10, with respect to a mass point A of the vibrating member 1, a counterclockwise revolving ellipsoidal motion defined by the longitudinal amplitude u and the transverse amplitude w is caused. A movable member 6 is maintained in contact with the surface of the vibrating member 1 by being pressed against the same. The movable member 6 contacts the vibrating member 1 at the apexes of the vibrating surface alone. (Actually, it contacts vibrating member surfaces of a certain width in the wave travel direction). The movable member 6 is driven by the longitudinal amplitude u component of the ellipsoidal motion of mass points A, AA, . . . at the apexes to move in the direction of the arrow N. When the 90.degree. phase shifter shifts the phase by +90.degree., the vibration wave travels in the direction -x, and the movable member 6 moves in the direction opposite to the direction N.
The AC voltage applied to such a vibration wave motor must be high enough to move the movable member 6, ordinarily several ten volts p--p or higher. Accordingly, for use in a small apparatus using ordinary dry batteries on the market, a means for boosting the voltage, e.g., a transformer is required.
To cope with this problem, the applicant of the present invention has already proposed a vibration motor having a stator constructed as shown in FIG. 11 (Japanese Patent Laid-Open No.59-96882). That is, the conductive vibrating member 1, an electrode 8A on the vibrating member side of the piezoelectric element 2a (hereinafter referred to as reverse electrode 8A), and an electrode 9A on the vibrating member side of the piezoelectric element 2b (hereinafter referred to as reverse electrode 9A) are electrically insulated from each other by insulating layer 7, and an AC voltage which is provided by inverting the AC voltage applied to electrodes 8, 9 on the side remote from the vibrating member (hereinafter referred to as obverse electrodes 8, 9) is applied to the reverse electrodes 8A, 9A. Theoretically, the same function and performance can be achieved by applying to this motor an AC voltage which is half the voltage required in the arrangement shown in FIG. 9. The insulator 7 which electrically insulates the reverse electrode 8A and 9A and the vibrating member 1 is unnecessary if the vibrating member 1 is not electrically conductive.
In the arrangement of FIG. 11, however, the thickness of the reverse electrodes 8A and 9A is very small (1 .mu.m or less) and it is therefore very difficult to apply AC voltages through end surfaces of the reverse electrodes.
Lead wires or flexible print plates or the like may be interposed between the reverse electrodes 8A and 9A to enable application of AC voltages to the reverse electrodes 8A and 9A. In this case, however, close contact between the vibration member i and the piezoelectric element 2 is impaired, resulting in a deterioration in motor performance, e.g., a reduction in efficiency.